Currently, carbon dioxide is a greenhouse gas that causes global warming, and should be reduced. Carbon dioxide may be reduced by means of trapping, chemical conversion or electrochemical conversion. Among them, the electrochemical conversion method may precisely control components so that other synthesis gases may be produced, and thus may give better economic benefits than merely removing carbon dioxide.
In general, an electrochemical reaction cell includes a cathode, an anode and an electrolyte. The electrolyte is classified into a solid electrolyte and a liquid electrolyte, and in some cases, both of solid and liquid electrolytes are used at the same time. The electrolyte may be classified into a cation exchange electrolyte and an anion exchange electrolyte, and a fluorine-based cation exchange membranes (Nafion) commonly used in fuel cells is a technically verified solid electrolyte.
In the case of the cation exchange membrane, which is widely used in the electrolysis of water as well as fuel cells, oxygen is generated when water is supplied to the anode, hydrogen is decomposed into electrons and protons, the protons move to a cathode through the cation exchange membrane, and the electrons move to the cathode through an external circuit to synthesize hydrogen gas.
Along with this reaction, if carbon dioxide (CO2) and KHCO3 electrolytic solution are supplied to the cathode, proton and carbon dioxide encounter each other to produce carbon monoxide and water. Since this is an electrochemical reaction, it is possible to easily control an amount of generated carbon monoxide and a hydrogen/carbon dioxide ratio by changing a voltage.
However, different from other fuel cells or water electrolytic cells, the electrochemical carbon dioxide reduction reaction requires a large contact area where the KHCO3 electrolytic solution and carbon dioxide (CO2) make contact with the catalyst at the same time. Thus, in the existing mixing and supplying method, there is a loss in the reaction area.
Thus, in the existing technique, KHCO3 electrolytic solution and carbon dioxide were mixed and supplied to increase a two-phase flow and a reaction area of the catalyst. However, in this case, liquid and gas are irregularly supplied to the cells due to surface tension, and if electrochemical reaction cells are stacked and used, the efficiency is remarkably lowered in comparison to the case where a single cell is used. In addition, in the existing technique, KHCO3 electrolytic solution and carbon dioxide were separately supplied and then mixed in a channel through a mesh made of a porous material. However, in this case, to control flow direction of carbon dioxide gas is difficult at a large area, so that the efficiency of the reduction reaction of carbon dioxide is lowered. In addition, in the existing technique, micro flows are controlled so that the KHCO3 electrolytic solution and carbon dioxide encounter at the catalyst layer without using the hydrogen gas exchange membrane. However, in this case, the cation exchange membrane is not used, so that it is difficult to increase the efficiency by pressurization, and it is difficult to maintain the micro flows in a large area. Thus, this is not available for mass-producing or stacking cells.
Therefore, there is required research for improving the reduction reaction of carbon dioxide by maximizing the a three-phase interface where KHCO3 electrolytic solution and carbon dioxide encounter a catalyst layer directly while using a cation exchange membrane.